This invention relates to information transmitting devices, and more particularly is directed to an information transmitting device which is applicable to devices which are designed as to record and reproduce desired information data on a magnetic tape.
An example of a conventional information data recording and reproducing device which records information data with high density is a so-called "ANSI ID-1 format" (Third Draft PROPOSED AMERICAN NATIONAL STANDARD 19 mm TYPE ID-1 INSTRUMENTATION DIGITAL CASSETTE FORMAT X3B6/88-12 Project 592-D 1988-03-22) magnetic recording and reproducing device. The ANSI ID-1 device records and reproduces desired information data, in contrast to the digital video data and digital audio data of a so-called "SMPTE D-1 format" digital video tape recorder (digital VTR) which records component digital video signals on a magnetic tape.
In a magnetic recording and reproducing device, as shown in FIG. 3, a magnetic tape 1 has a time code track TC and a control track CTL which are extended longitudinally along the lower edge of the magnetic tape 1. The time code track TC is used for editing, and the control track CTL is used for tracking control. A magnetic head helically scans the magnetic tape 1 obliquely, thus forming recording tracks TR (. . . , TR1, TR2, TR3, TR4, . . . ) which alternately differ in their azimuth angles.
In practice, each recording track TR corresponds to one sector, SEC, or record unit, as shown in FIG. 4, and is comprised of a preamble part PR, a record data part DT, and a postamble part PS.
The preamble part PR includes a rise sequence RUS 20 bytes in length in which is written a clock signal for the phase-lock loop (PLL) circuit in a read circuit, a synchronizing code SYNC.sub.PR 4 bytes in length, sector identifying data ID.sub.SEC1 4 bytes in length, and extended data DT.sub.AUX 6 bytes in length which is provided for user data.
The record data part DT consists of 256 data blocks BLK (BLK0, BLK1, BLK2, . . . , and BLK255). Each data block BLK includes a block synchronizing code SYNC.sub.BLK 4 bytes in length, block identifying data ID.sub.BLK 1 byte in length, information data DT.sub.INF 153 bytes in length, and a Reed-Solomon code RSC.sub.BLK 8 bytes in length for error detection and correction of block identifying data ID.sub.BLK and information data DT.sub.INF.
The postamble part PS consists of a synchronizing code SYNC.sub.PS 4 bytes in length, and sector identifying data ID.sub.SEC2.
When using the magnetic recording and reproducing device, it is necessary to reproduce the 4-byte sector identifying data ID.sub.SEC1, and the 6-byte extended data DT.sub.AUX in the preamble part PR prior to reproducing the information data DT.sub.INF recorded in the record data part DT.
For this purpose, the 4-byte synchronizing code SYNC.sub.PR succeeding the 20-byte rise sequence RUS is usually detected, and then using the time of such detection as a reference, the following 4-byte data and 6-byte data are reproduced as the sector identifying data ID.sub.SEC1 and the extended data DT.sub.AUX, respectively.
Consequently, it is essential to correctly detect the synchronizing code SYNC.sub.PR in the preamble part PR. If that synchronizing code is not correctly detected, then it will be difficult to reproduce the sector identifying data ID.sub.SEC1 and the extended data DT.sub.AUX.
In practice, in each record track TR, the preamble part PR is the location where the helically scanning magnetic head comes into contact with the magnetic tape. Hence, the preamble part PR has a higher probability than the data record part DT located in the middle of the tape 1 of experiencing a tracking or other error.
The ratio of the byte length of the synchronizing code SYNC.sub.PR to the byte length of the entire preamble part PR is 4/14. When an error occurs within the preamble part PR, it is more likely than not that such error will occur at the beginning thereof where the synchronizing code SYNC.sub.PR is situated; that is, the probability is high that, in the event of an error in the preamble part PR, the synchronizing code SYNC.sub.PR is not correctly detected. In such case, the sector identifying data ID.sub.SEC1 and the extended data DT.sub.AUX are frequently detected incorrectly.
Even if the synchronizing code SYNC.sub.PR of the preamble part PR is correctly detected, its correctness cannot be determined because no error detecting/correcting code is employed. Therefore, the erroneous part may be detected as the synchronizing code SYNC.sub.PR, and this in turn increases the probability is increased as much that the sector identifying data ID.sub.SEC1 and the extended data DT.sub.AUX are erroneously reproduced.
In order to overcome this problem, a method has been proposed in which, if an error occurs in reading the sector identifying data from a sector, but the preceding sectors are correct, then it is assumed that the sector in question is also correct (Japanese Patent Application Laid-open No. 251564/1985).
However, the above-mentioned method may cause a serious error in the reproduction of information; that is, it is still insufficient to solve the above-described problem.